1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device with a verify function, and more particularly to a nonvolatile semiconductor memory device with memory cells for verification.
2. Description of the Related Art
With nonvolatile memories, such as EEPROMs or flash memories, it is necessary to verify whether the data has been written or erased correctly after the writing or erasing of the data. The verifying operation is termed verify. Verify is executed by comparing the current from the cell that has been written into or erased from with the current from a reference cell at a sense amplifier. In general, a write operation or an erase operation is carried out during a specific period of time and thereafter, a verify operation is executed. After the repetition of these operations has brought the current value of the cell into a specified value, the verify operation is terminated and the writing or erasing is completed.
Although the expression "the writing or erasing" is used here, a write operation is generally independent from an erase operation. In this connection, verify associated with a write operation is termed program verify, whereas verify related to an erase operation is termed erase verify.
As the degree of integration of flash memory is getting higher, the proportion of verification in a write or erase operation is getting larger, resulting in an increase in the time required to write or erase the data. Program verify, in particular, is a factor that hinders the reduction of the write time, because being executed bit by bit.